<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
    <style type="text/css">
      * {
        padding: 0;
        margin: 0;
      }

      li {
        list-style: none;
      }

      body {
        background: #f6f9fc;
        font-family: arial;
      }

      .calendar {
        width: 210px;
        margin: 0 auto;
        padding: 10px 10px 20px 20px;
        background: #eae9e9;
      }

      .calendar ul {
        width: 210px;
        overflow: hidden;
        padding-bottom: 10px;
      }

      .calendar li {
        float: left;
        width: 58px;
        height: 54px;
        margin: 10px 10px 0 0;
        border: 1px solid #fff;
        background: #424242;
        color: #fff;
        text-align: center;
        cursor: pointer;
      }

      .calendar li h2 {
        font-size: 20px;
        padding-top: 5px;
      }

      .calendar li p {
        font-size: 14px;
      }

      .calendar .active {
        border: 1px solid #424242;
        background: #fff;
        color: #e84a7e;
      }

      .calendar .active p {
        font-weight: bold;
      }

      .calendar .text {
        width: 178px;
        padding: 0 10px 10px;
        border: 1px solid #fff;
        padding-top: 10px;
        background: #f1f1f1;
        color: #555;
      }

      .calendar .text h2 {
        font-size: 14px;
        margin-bottom: 10px;
      }

      .calendar .text p {
        font-size: 12px;
        line-height: 18px;
      }
    </style>
  </head>

  <body>
    <div id="tab" class="calendar">
      <ul id="ul">
        <li class="active">
          <h2>1</h2>
          <p>JAN</p>
        </li>
        <li>
          <h2>2</h2>
          <p>FER</p>
        </li>
        <li>
          <h2>3</h2>
          <p>MAR</p>
        </li>
        <li>
          <h2>4</h2>
          <p>APR</p>
        </li>
        <li>
          <h2>5</h2>
          <p>MAY</p>
        </li>
        <li>
          <h2>6</h2>
          <p>JUN</p>
        </li>
        <li>
          <h2>7</h2>
          <p>JUL</p>
        </li>
        <li>
          <h2>8</h2>
          <p>AUG</p>
        </li>
        <li>
          <h2>9</h2>
          <p>SEP</p>
        </li>
        <li>
          <h2>10</h2>
          <p>OCT</p>
        </li>
        <li>
          <h2>11</h2>
          <p>NOV</p>
        </li>
        <li>
          <h2>12</h2>
          <p>DEC</p>
        </li>
      </ul>

      <div class="text" id="txt">
        <h2 class="hhh">1月活动</h2>
        <p>快过年了，大家可以商量着去哪玩吧～</p>
      </div>
    </div>

    <script>
      var Oh2 = document.getElementsByClassName("hhh")[0];
      
      var Oli = document.getElementsByTagName("li");
      for (var i = 0; i < Oli.length; i++) {
        Oli[i].index = i+1;
        
        Oli[i].onclick = function () {
          for (var j = 0; j < Oli.length; j++) {
            Oli[j].className = "";

            this.className = "active";
            
          }
          Oh2.innerHTML=this.index+"月活动"
        };
      }
    </script>
  </body>
</html>
